US President Donald Trump has called Russian President Vladimir Putin to offer his condolences after a blast on the St Petersburg Metro killed at least 11 people on Monday (3 April).
Russian Prime Minister, Dmitry Medvedev called the explosion that injured a further 45 people a "terrorist attack".
A readout of the call from the White House said that Trump "expressed his deepest condolences to the victims and their loved ones, and to the Russian people."
It went on to say that Trump "offered the full support of the United States Government in responding to the attack and bringing those responsible to justice."
The two leaders then agreed that "terrorism must be decisively and quickly defeated."
A Russian news agency said that investigations were focused on a 23-year-old man from Central Asia.
Earlier on Monday, before a meeting with Egyptian President Abdel Fattah el-Sisi, Trump said that the explosion was "terrible. Terrible thing." Adding: "Happening all over the world. Absolutely a terrible thing."
The cause of the explosion is not yet known though the reported discovery of an exposive device at another metro station which was defused suggested a coordinated attack.